NASA and Boeing have called off this week’s planned Feb 25 second test fire of the space agency’s mammoth SLS moon rocket core stage, following inspections and checkouts over the weekend which discovered one of eight valves (called a prevalve) not working properly. The valves are part of the core stage’s main propulsion system that supplies liquid oxygen to each of four RS-25 engines.
The latest delay comes following a much shorter than anticipated debut test fire of 212 foot tall core stage just weeks ago at Stennis Space Center in Bay St Louis, Mississippi. The planned 8-minute test fire barely made it past 1-minute, before an automatic shutdown was triggered by intentionally conservative test parameters, according to a NASA.
“During the first hot fire, all four liquid oxygen valves performed as expected as did the four liquid hydrogen valves,” says NASA. “NASA and the core stage lead contractor Boeing will identify a path forward in the days ahead and reschedule the hot fire test that was originally scheduled for Feb. 25.”
“This is the biggest test NASA has run in 50 years,” said SLS Program Manager John Honeycutt of the Marshall Space Flight Center (MSFC) in Huntsville, Ala. He underlined the intentional conservatism of the test parameters following the first test fire, since “this is not just a test article, it’s the flight article”.
As outlined in detail previously on AmericaSpace by Ben Evans, the SLS Hot Fire Test is the eighth and final step in the “Green Run”, a year-long campaign to wring out the Core Stage’s myriad systems ahead of the rocket’s maiden voyage and the uncrewed Artemis-1 mission around the Moon, possibly as soon as late 2021 or early 2022.
Five “functional” tests to validate the rocket’s Guidance, Navigation and Control (GNC) systems, evaluate its avionics and safety systems and check out its Main Propulsion System (MPS), Thrust Vector Control (TVC) and hydraulics were completed between January and September 2020. Satisfactory completion of these steps allowed Stennis teams to press into three “operational” tests, beginning last fall, which saw the Core Stage put through a mock countdown, fueled with its full load of propellants in a so-called “Wet Dress Rehearsal” (WDR) and all four RS-25 engines hot-fired.
Original plans called for the four engines—all of which are refurbished Space Shuttle Main Engines (SSMEs), with over 1.1 million seconds’ worth of “burn-time” and a total of 25 shuttle missions to their credit—to be fired for up to 485 seconds, approximating as closely as possible the conditions that they will encounter during the raging, eight-minute climb to orbit on a real mission.
To mimic the passage through a period of maximum aerodynamic turbulence (“Max Q”), about a minute after liftoff, the RS-25s were to be throttled back from their maximum 109-percent thrust level to 95 percent for about 30 seconds, then returned to full power. It was also expected that the engines would be “gimbaled” under TVC control to demonstrate their steering capabilities.
As the first test fire got underway and all four engines came alive, the first minute of stable thrust proceeded without incident. Then at 60 seconds, the pre-planned gimbaling test of the engines under TVC control got underway. Responsibility for gimbaling each engine fell to the TVC actuators, each powered by a Core Stage Auxiliary Power Unit (CAPU).
At approximately 61 seconds, CAPU-2—serving the Core Stage’s No. 2 engine—detected low hydraulic fluid levels and after a series of verification checks over the next two or three milliseconds to validate this reading, it shut itself down. The other three CAPUs momentarily increased their hydraulic pressures to 105 percent to compensate for this evolving situation. CAPU-2 then commanded the Core Stage flight computer to shut down the other engines. This was executed safely over the next few seconds and the Hot Fire Test ended after 67.2 seconds, which represented less than 15 percent of a full-flight-duration burn.
Summing up the first test fire, NASA noted that—had it been a “real” flight—the CAPU margins would have been higher and CAPU-2 would have continued to function nominally. “The specific logic that stopped the test is unique to the ground test, where the Core Stage is mounted in the B-2 Test Stand at Stennis,” NASA explained. “If this scenario occurred during a flight, the rocket would have continued to fly using the remaining CAPUs to power the Thrust Vector Control systems for the engines.”